Fuel cells can be used as a power source in many applications. For example, fuel cells have been proposed for use in automobiles as a replacement for internal combustion engines. In proton exchange membrane (PEM) type fuel cells, a reactant such as hydrogen is supplied as a fuel to an anode of the fuel cell, and a reactant such as oxygen or air is supplied as an oxidant to the cathode of the fuel cell. The PEM fuel cell includes a membrane electrode assembly (MEA) having a proton transmissive, non-electrically conductive, proton exchange membrane. The proton exchange membrane has an anode catalyst on one face and a cathode catalyst on the opposite face. The MEA is often disposed between “anode” and “cathode” diffusion media or diffusion layers that are formed from a resilient, conductive, and gas permeable material such as carbon fabric or paper. The diffusion media serve as the primary current collectors for the anode and cathode as well as providing mechanical support for the MEA and facilitating a delivery of the reactants.
In a fuel cell stack, a plurality of fuel cells is aligned in electrical series, while being separated by gas impermeable, electrically conductive bipolar plates. Each MEA is typically sandwiched between a pair of the electrically conductive plates that serve as secondary current collectors for collecting the current from the primary current collectors. The plates conduct current between adjacent cells internally of the fuel cell stack in the case of bipolar plates and conduct current externally of the stack in the case of unipolar plates at the ends of the stack.
The bipolar plates typically include two thin, facing conductive sheets. One of the sheets defines a flow path on one outer surface thereof for delivery of the fuel to the anode of the MEA. An outer surface of the other sheet defines a flow path for the oxidant for delivery to the cathode side of the MEA. When the sheets are joined, a flow path for a dielectric cooling fluid is defined. The plates are typically produced from a formable metal that provides suitable strength, electrical conductivity, and corrosion resistance, such as 316 alloy stainless steel, for example.
The bipolar plates have a complex array of grooves or channels that form flow fields for distributing the reactants over the surfaces of the respective anodes and cathodes. Tunnels are also internally formed in the bipolar plate and distribute appropriate coolant throughout the fuel cell stack, in order to maintain a desired temperature.
The typical bipolar plate is a joined assembly constructed from two separate unipolar plates. Each unipolar plate has an exterior surface having flow channels for the gaseous reactants and an interior surface with the coolant channels. Plates are known to be formed from a variety of materials, including, for example, a metal, a metal alloy, or a composite material. The metals, metal alloys, and composite materials must have sufficient durability and rigidity to function as sheets in the bipolar plate assembly, as well as to withstand clamping forces when assembled into a fuel cell stack without collapsing. It is known to form the plates using various processes such as, for example, machining, molding, cutting, carving, stamping, or photo-etching. In each known method of forming the plates, a substrate material, typically a metal or composite sheet, is required. It is possible to achieve a desired minimal thickness of the substrate, but at a tradeoff to cost and to undesirable material properties. For example, as a composite sheet is molded to a thinner dimension, it becomes more brittle and harder to work. Additionally, a thinner composite sheet is often less desirable because high carbon content may cause a thinner sheet to become porous. Similarly, as a metal sheet is thinned in multiple steps by drawing or rolling the sheet, it also becomes brittle or work hardened after each step, and requires annealing prior to further working. Thus, a higher manufacturing cost is associated with a thinner substrate material. Also, more care is required to form the complex surface features of the plates, such as the flow field pattern, from a thinner metal substrate material to avoid localized areas of high stress and the resulting cracks or tears in the plates due to thinner material. A thinner metal substrate also limits the depth of any flow channel due to metal stretch limitations. As a result, metal sheet plates are optimally formed having a thickness of about 3 to 6 mils (0.003 to 0.006 inches, or approximately 0.075 to 0.15 millimeters thick). It is understood, however, that thicker metal plates may be employed thicker in order to reduce cost and to improve workability of the plate material.
Additionally, conventional processes of forming the plates from the metal sheet material result in nearly half of the material being discarded as scrap. Some of the scrap is generated as apertures are punched in the non-active portion of the plates to create flow areas and manifolds for delivery and exhaust of reactants and coolant when a plurality of bipolar plates is aligned in the fuel cell stack. A larger portion of the scrap results from a clamping area that is required about the perimeter of the sheet material during the processes that form plates from the sheet material, which is then trimmed or cut off after processing.
Finally, in order to conduct electrical current between the anodes and cathodes of adjacent fuel cells in the fuel cell stack, the paired unipolar plates forming each bipolar plate assembly are mechanically and electrically joined. A variety of bipolar plate assemblies and methods for preparing bipolar plate assemblies are known in the art. For example, it is reported by Neutzler in U.S. Pat. No. 5,776,624, incorporated herein by referenced in its entirety, that a bipolar plate including corrosion-resistant metal sheets may be brazed together to provide a passage between the sheets through which a dielectric coolant flows. Further, U.S. Pat. No. 6,887,610 to Abd Elhamid, et al., incorporated herein by reference in its entirety, discloses a bipolar plate assembly without welding or brazing that includes an electrically conductive layer deposited over the coolant channels and lands and a fluid seal disposed between the inside facing surface about a perimeter of the coolant channels. Also, U.S. Pat. No. 6,942,941 to Blunk et al., incorporated herein by reference in its entirety, recites a bipolar plate having a first and second surface that are coated with an electrically conductive primer coating and joined to one another by an electrically conductive adhesive. Schlag in U.S. Pat. No. 7,009,136, incorporated herein by reference in its entirety, describes a method of fabrication adapted to weld bipolar plates together using a partial vacuum that holds paired unipolar plates together during the welding process. Commonly owned U.S. Pat. Appl. Pub. No. 2008/0292916, incorporated by reference herein in its entirety, discloses a bipolar plate assembly that includes a first unipolar plate disposed adjacent a second unipolar plate, where the first and second unipolar plates are bonded together by a plurality of localized electrically conductive nodes. The bonds may be formed as a weld, a solder joint, a braze joint, and an adhesive.
There is a continuing need for a cost-effective bipolar plate assembly having an efficient and robust structure that provides an optimized electrical contact between the plates of the assembly while minimizing material usage and waste and maximizing the structural integrity of the plates. A method for rapidly producing the bipolar plate assembly applicable to optimized flowfield designs is also desired.